DRAGNET Cluster Usage

Hope this helps… If not let me (Alexander) know (amesfoort@astron.nl).

To get an account, ask Jason Hessels (hessels@astron.nl).
With permission from Jason, ask Teun Grit (grit@astron.nl) to add access to DRAGNET (via NIS). If you don't have access to the LOFAR portal, tell him. Idem for the ASTRON portal, i.e. if you are not working for ASTRON.

Having an account, ssh to hostname dragnet.control.lofar or easier, just dragnet, from the LOFAR portal (or tunnel through it):

ssh USERNAME@dragnet

Within the cluster (or even to it), don't bother typing your password all the time. Passwords make cluster-wide commands a nightmare. Instead, use an ssh key pair:

ssh-keygen -t rsa  # or copy an existing public key pair to .ssh/
cat .ssh/id_rsa.pub >> .ssh/authorized_keys
chmod 600 .ssh/authorized_keys

Now test if this works by logging in and out to drg01 without entering a password (this should succeed with no output):

ssh drg01 exit

(For completeness: Your .ssh/id_rsa contains your private key. Do not share it with others. If compromised, asap regenerate the key pair.)

If you are just running some computations on DRAGNET, skip this section. But if you need fast networking, or are already deep in the slow data transfers and rapid-fire connection errors, here is some info that may save you time wrt the multiple networks and network interfaces. (Or just tell us your needs.)

• dragnet(.control.lofar)
• dragproc(.control.lofar)
• drg01(.control.lofar) - drg23(.control.lofar)

Control/Management network: NODENAME.control.lofar (1 Gb) (all nodes)
10G network:                NODENAME-10g.online.lofar (10 Gb) (all drgXX nodes and ''dragproc'')
Infiniband network (IPoIB): NODENAME-ib.dragnet.infiniband.lofar (56 Gb) (all drgXX nodes)

(There is also a 1 Gb IPMI network.)

When going cross-cluster, prefer to use the fully-qualified domainnames (FQDN), esp. in scripts or programs (i.e. drg11-10g.online.lofar instead of just drg11). See /etc/hosts on any node for the list.

In most cases, you will use the network as deduced from the destination hostname or IP. Indicate a 10G name to use the 10G network. Idem for infiniband (IPoIB). (Exception: CEP 2, see below.)

Note: Copying large data sets at high bandwidth to/from other clusters (in particular CEP 2) may interfere with running observations as long as CEP 2 is still in use. If you are unsure, ask us. It is ok to use potentially oversubscribed links heavily, but please coordinate with Science Support!

Initiate connections for e.g. data transfers from CEP 2 to HOSTNAME-10g.online.lofar and you will go via 10G.

The reverse, connecting from DRAGNET to CEP 2, by default will connect you via DRAGNET 1G (e.g. for login). To use 10G (e.g. to copy datasets), you need to bind to the local 10G interface name or IP. The program you are using has to support this via e.g. a command-line argument.

To run a command over many cluster nodes, use cexec (as on CEP2/3), ansible, or a shell loop around an ssh/scp command. (First, see the section above on Password-less Login.)

• cexec (shell) runs any shell command in parallel. Output is sorted and only appears after all nodes finished. Indexed hostname specification.
• ansible (Python) is easy with simple commands or with Ansible modules to support idempotent changes. Easy integration in Python programs. No sorted output, but node output appears when a node is done. No shell interpretation of commands, which may be a restriction or rather safe. Can run commands in parallel. Tailored for system administration, configuration and deployment.
• shell loop around ssh is most basic and possibly powerful wrt UNIX tools, but tricky wrt escaping, which remote environment values are actually used, and for dealing correctly with filename corner cases. Scripts easily end up shell specific (e.g. bash vs tcsh).

NOTE: be careful with potentially destructive operations like rm -rf. Accidents have happened (data loss) on CEP2 with cexec and shell scripts.

The Cluster Command and Control (C3) tool suite contains the cexec(1) program that can be used to run commands over many nodes.

Example:

cexec drg:3-5 "df -h"    # disk usage on the drg04(!), drg05, drg06(!) nodes
cexec dragnet:23 ls      # run ls on dragproc
cexec hostname           # hostnames as seen from each cluster node

The hostname specifier (2nd optional argument) must contain a ':' and may also be drg, which excludes the dragproc node. The dragnet hostname specifier contains all nodes (incl head node). The drg group is without dragproc. (It is not possible to define a C3 group without the head node.) Note that the hostname numbers here specify start and end index (starting at 0!).

Ansible is a tool to automate cluster (administration) tasks.

Examples of simple commands:

ansible alldragnet -a 'df -h'                                  # disk usage on all nodes
ansible 'proc;workers' -f 25 -a 'df -h /data1 /data2'          # disk usage on dragproc and worker nodes, connect to max 25 nodes at a time
ansible workers -f 25 -a 'ls -al /data1/LOBSID /data2/LOBSID'  # list /data*/LOBSID files on all drg* nodes, connect to max 25 nodes a time
ansible 'drg01;drg17' -a 'ls -l /data1'                        # list /data1 on drg01 and drg17

Apart from hostnames, the following hostname groups are recognized on DRAGNET: head, proc, workers, alldragnet.

The command must be a simple command. It can be the name of an executable shell script if accessible to all hosts, but not a compound shell command with &, &&, pipes or other descriptor redirection (you can of course run the shell with some argument, but then, what's the point of using ansible?).

Background: Ansible heavily relies on the idea to specify what you want in terms of the desired situation rather than what to do to get there. Such idempotent commands work correctly regardless whether some nodes are already ok or different. To this end ansible has numerous modules to manipulate system settings in an easy way, but you can also write your own modules (e.g. to reinstall (parts of) a type of node), or so-called playbooks to manage configuration and deployment.

For many common system admin related tasks, use an ansible module. Search the Ansible Module Index for more info.

Examples:

for ((i = 1; i <= 10; i++)); do host=$(printf drg%02u $i); ssh $host "df -h"; done  # disk usage on the drg01-drg10 nodes
for host in drg01 drg17; do ssh $host "df -h"; done                                 # disk usage on drg01 and drg17

Be careful with complex commands!

To utilize the cluster efficiently, we use the SLURM workload manager. This is also supposed to ensure that batch jobs do not interfere with observations that DRAGNET participates in (as in: micromize observation data loss).

Random notes:

• SLURM does not enforce accessing nodes through it; one can access any node via ssh. Depending on the intention and the current workload, that may be fine or less desirable.
• SLURM has a ton of options that we haven't all set up. In particular, atm it does not enforce exclusive access to GPUs via cgroups (although it does set CUDA_VISIBLE_DEVICES if you explicitly request GPUs). Once a node is (partially) assigned to your program, your program can in principle use any resource on that node.

If you are having trouble using SLURM, please contact Alexander (amesfoort@astron.nl).

From any DRAGNET node (typically the dragnet head node), you can submit compute (or perhaps also separate data transfer) jobs.

Run a single task, see output as it is produced, and wait for completion. Note that in this case the ls program must be available on any node that may be used.

$ srun -n 1 ls dir1 dir2
file1
file2
[...]

Show list of jobs queued:

$ squeue
           JOBID PARTITION     NAME     USER ST       TIME  NODES NODELIST(REASON)
               9   workers       ls amesfoor CD       0:01      1 drg

Show list of recently completed jobs:

$ squeue -t COMPLETED
           JOBID PARTITION     NAME     USER ST       TIME  NODES NODELIST(REASON)
               9   workers       ls amesfoor CD       0:01      1 drg

Show list and state of nodes. When submitting a job, you can indicate one of the partitions listed or a (not necessarily large enough) set of nodes that must be used. Please hesitate indefinitely when trying to submit insane loads to the head partition. :)

$ sinfo
PARTITION AVAIL  TIMELIMIT  NODES  STATE NODELIST
workers*     up   infinite     23   idle drg[01-23]
proc         up   infinite      1   idle dragproc
head         up   infinite      1   idle dragnet

If you get an error on job submission that there are no resources in the cluster to ever satisfy your job, and you know this is wrong (no typo), you can see with the sinfo if there are nodes out of service. (SLURM may remove a node from a partition on misconfiguration or hardware malfunctioning.)

The sbatch(1) command offers to:

• take a user-supplied job (batch) script, not just to start your script, but also to set up a job array or workflow
• have stdout/stderr go to a file
• copy the program (and possibly library and data dependencies) to the to be used nodes
• run the job without blocking your terminal on its completion. This is useful for e.g. substantial processing jobs
• auto-restart on failure (not sure when/how that applies)

Apart from nodes, it is also possible to indicate scheduling constraints on CPU cores, GPUs, memory, or network bandwidth (if we set that up).
Atm, you have to indicate constraints for:

• either number of nodes or CPUs
• number of GPUs, if any needed. If no GPUs are requested, any GPU program will fail. (Btw, this policy is not fully as intended, so if technically it can be improved, we can look into it.)

You do not have to indicate memory size, but if you don't, SLURM will grant you all the memory of a node, preventing other jobs from running on the same node(s). This may or may not be the intention. (If the intention, better use --exclusive.)

Note that a CPU is to SLURM a hardware resource that the OS can schedule a task on. This is typically a hardware thread, or if no hyperthreading, a CPU core.

To indicate a scheduling resource constraint on 2 GPUs, use the –gres option (gres stands for generic resource):

$ srun --gres=gpu:2 -n 1 your_gpu_prog

To indicate a list of nodes that must be used (list may be smaller than number of nodes requested). Some examples:

$ srun --nodelist=drg02 ls
$ srun --nodelist=drg05-drg07,drg22 -n 8 ls
$ srun --nodelist=./nodelist.txt ls   # with a '/' in the arg value

For the moment, see more explanation and examples at http://www.umbc.edu/hpcf/resources-tara-2013/how-to-run.php

Please see the manual pages on srun(1), sbatch(1), salloc(1) and the SLURM website for more info.